Single-ended thyratron discharge device



May 22, 1956 H. N.,PR|CE SINGLE-ENDED THYRATRON DISCHARGE DEVICE Filed June 5, 1951 Inventor: Harvey N. -Price, m 8% His Attorney.

United States Patent SINGLE-ENDED THYRATRON DISCHARGE DEVICE Harvey N. Price, Schenectady, N. Y., assignor to General Electric Company, a corporation of New York Application June 5, 1951, Serial No. 229,931

9 Claims. (Cl. 313-38) My invention relates to electric discharge devices of the vapor or gas filled controlled rectifier type, commonly known as thyratrons, and more particularly to singleended thyratron discharge devices in which the connectors for the electrodes extend through only one end of the envelope.

' Controlled rectifiers have for the most part in the past been of the so-called double-ended construction, in which the connectors for the electrodes extend through both ends of the envelope. Usually, the anode connector is extended through the top of the envelope to an external terminal, while the remaining electrode connectors extend through the bottom of the envelope. With the object of reducing the required mounting height and providing more convenience in connecting external lead wires to the connectors, thyratron discharge devices are now being made with single-ended construction, i. e., with all electrode connectors extending through one end of the envelopes of the devices. This single-ended construction requires that the supporting and connecting leads for the anode of the device pass in close proximity to the cathode and control grid of the device, and, even though insulated from the control grid, also be spaced therefrom to prevent grid-anode leakage current. In this arrangement, it has been found that the control of the control grid over the initiation of conduction has been inadequate mainly because ionization breakdown paths from cathode to anode potential exist which do not include the control grid wires and therefore are not controlled by the control grid potential. The undesirable situation of a cathode-to-anode potential ionization path not including the control grid wires may result from the spacing required between the anode leads and the control grid, or from the deposition of emissive or conductive material vaporized from the cathode on insulators in proximity to the cathode and in contact with anode leads at anode potential. it has also been found that long surface insulators employed in single-ended thyratrons may result in undesired ionization discharge at points remote from the cathode-anode space unless means are provided to prevent positive ion charge from the gaseous medium from collecting on the surfaces of the insulators and being transferred at anode potential to a point where a long path to cathode potential exists and undesirable breakdown may occur.

It is an object of my invention to provide a new and improved electric discharge device of the thyratron type.

It is another object of my invention to provide an electric discharge device of the single-ended thyratron type in which there is no loss of grid control due to ionization paths between the cathode and points of anode potential which do not include the control grid wires.

It is another object of my invention to provide an electric discharge device of the single-ended thyratron type in which the condensation of vaporized cathode emitting or conducting material on critical portions of insulator surfaces is reduced.

It is still another object of my invention to provide 2,747,126 Patented May 22, 1956 means in a single-ended thyratron which prevent the buildup of surface charge on insulators and resulting spurious long path ionization discharges.

Briefly stated, my invention in one form thereof is embodied in a single-ended thyratron discharge device comprising an hermetically sealed envelope, containing an ionizable medium either vapor or gas, in which are located an emissive cathode surrounded by a heat shield having a single opening therethrough, an anode, and a control grid. The control grid and anode are supported in spaced relation from the shield opening, in the order named, to control the initiation of current flow and receive current flow from the emissive cathode respectively. The control grid and the anode are each supported and electrically connected by two lead members which extend along the sides of the heat shield, being insulated therefrom by tubular insulators. The tubular insulators are held and shielded by shielding straps which are formed therearound and fastened to the outer side of the heat shield along its entire length. .Since the two insulated lead members supporting the anode extend past the location of the control grid and in order to prevent undesirable leakage current between the control grid and the anode, the control grid is formed to have recesses or indentations defining spacing between itself and the insulated anode lead members. To keep the indentations from providing a rather direct path from the cathode to the anode, over which ionization and conduction could otherwise occur even when the control grid is highly negative, means'including a grid barrier or gusset is fastened around each indentation, extending toward the cathode and heat shield. Also, these means include a cathode barrier or wall which is fastened to the heat shield, intermediate the opening therein and each of the gussets extending toward the control grid. Discharge and initiation of discharge is thus restricted to a path including the central grid portion and initiation of discharge or ionization cannot occur with a reasonably negative potential on the control grid through the grid indentations because of the labyrinthine indirect path formed by the gussets and the cathode barriers. Further, the gussets and the cathode barriers restrict discharges in the device to the central portion of the control grid, so that the insulators on the anode lead members receive little, if any, coating of conducting or semi-conducting material, due to condensation of evaporated cathode emissive or conductive material which might transfer anode potential along the insulator surfaces to within such proximity of the cathode that ionization could be initiated between the cathode and the anode lead insulators. These barrier means, therefore, provide a singleended thyratron which is not limited by poor grid control.

The novel features of my invention are pointed out with particularity in the appended claims. For a better understanding of my invention, however, together with further objects and advantages thereof, reference should be had to the following description taken in conjunction with the accompanying drawing in which:

Fig. 1 is a side elevation in section of an electric discharge device embodying my invention; Fig. 2 is a sectional view of the device taken along line 22 in Fig. l; and Fig. 3 is a sectional view of the device taken along line 33 in Fig. 1.

Referring now to the drawing, I have shown an electric discharge device of the thyratron type having an hermetically sealed envelope 1 which contains therein an ionizable medium, such as mercury vapor, xenon, argon, or others well known in the art, under low pressure in the order of 70 to microns. Extending in sealed and mutually insulated relation through the base of envelope 1, there are a plurality of lead-in prongs or connectors 2-7 which provide external terminals and structural support for the electrodes of the device.

A cathode 8, made of a cylindrical stagger-slotted metal sheet coated with a suitable thermionically emissive material, such as a mixture of nickel oxide and barium or other well-known emissive substances, is secured at its top end to a double-walled heat shield 9 by means of a connecting collar 10. Cathode 8 is shown by way of example as a suitable direct-heated cathode, the staggered slotting therein providing a plurality of heating current paths in parallel. This cathode construction is more completely shown and described in U. S. Patent 2,111,506-Edwards, to which reference may be had.

Heat shield 9, which forms a heat shielding tank or cylinder surrounding cathode 8, includes two concentric cylinders 11 and 12 which thus provide two heat-reflecting surfaces. Cylinders 11 and 12 may be made of nickel, for example, and preferably are made very thinin the order of .003 to .005 inch-to reduce the thermal mass thereof and provide a cathode structure which heats quickly to thermal equilibrium at the proper cathode operating temperature. 1

The bottom end of heat shield 9 is closed by a double wall, not visible in the drawing, while the top end is provided with two annular headers 13 and 14, defining an opening 15 therein through which electric discharge may take place. Cathode 8 is connected at its bottom end by a cathode lead 16, collared by an insulator 17 in the region passing through the bottom end of shield 9. Lead 16 in turn is connected to connector by means of a preferably wide metal tab 18 which possesses enough radiating surface to assure that the lower end of lead 16 does not become hot enough to be emissive and cause undesirable discharge between it and connectors at anode potential. Heat shield 9 is further supported and electrically connected by a lead 19 welded from it to connector 7 as shown. A circuit for cathode heating current is thus provided by connector 5, lead 16, cathode 8, shield 9, lead 19, and connector 7.

Mounted in spaced relation from the opening in the top of heat shield 9 by means of at least two anode leads 2i) and 21, is a circular disk anode 22 which may be made of tantalum or nickel and provided with an upwardly extending rim portion 23 and a center depressed portion 24 for increased rigidity. Tabs 25 are welded to rim portion 23 and also to leads 20 and 21 as shown, to accomplish this mounting. Anode leads 20 and 21 extend from the bottom end of envelope 1, where they are bonded to connectors 2 and 6 respectively, in insulated relation to heat shield 9 along the sides of the heat shielding tank or cylinder as shown. ried out by providing two tubular insulators 26 and 27, preferably of a ceramic material such as aluminum oxide, around anode leads 20 and 21, in order that the spacing between the outer surface of heat shield 9 and leads 20 and 21 be kept small and the total required diameter of the device minimized. Shielding and fastening straps 28 and 29 are provided around insulators 26 and 27 extending substantially the whole length of heat shield 9, to prevent the collection of charge on the insulator surfaces and a long path discharge, as will be explained hereafter, and fastened by welding to the outer surface of the heat shielding tank.

A control grid, interspaced between anode 22 and opening 15, is provided by a plurality of grid wires 30 welded in spaced relation as shown, across an opening 31 in an annular cup-like member 32. Member 32 may conveniently be made of nickel, for example, and formed with a network of ribs 33 thereon to enhance its rigidity. Grid wires 30 may be made of pure nickel but are preferably made of tungsten, which is oxidized to form an external emission inhibiting tungsten oxide layer 34 thereon, to prevent cold grid emission from emissive material which may be vaporized from the cathode and condensed on the grid wires during operation of the device. The oxide layer 34 is cleaned from the extremities of grid This is carwires 30 as shown in order that they may be welded to member 32. Member 32 is supported and electrically connected by means of two grid leads 35 and 36, which are circumferentially spaced from leads 20 and 21, which extend in parallel and insulated relation to heat shield 9 from the bottom of envelope 1 where they are welded to connectors 4 and 3 respectively, and which are welded to the rim of member 32. Tubular insulators 37 and 38, preferably ceramic, are provided around leads 35 and 36 and two shielding and fastening straps 39 and 40 are provided around insulators 37 and 33, extended substantially the entire length of heat shield 9 and bonded to the outer wall of the heat shielding cylinder 12 as illustrated.

Annular cup-like member 32 must necessarily be larger in diameter than heat shield 9 in order to confine electric discharge to a path including grid wires 30 and make the control grid potential effective in controlling the initiation of discharge. Further, it is desirable that cup-like member 32 be very nearly the same diameter as envelope 1 to allow good heat transfer therefrom to envelope 1 and cooler operation of the control grid, and to reduce the possibility of uncontrolled discharge from cathode 8 to anode 22 around the edges of member 32. It is therefore necessary that anode leads 20 and 21 pass through openings in member 32; in fact, it has been found necessary to provide spacing between insulators 26 and 27 and the control grid member 32 to prevent control grid-toanode leakage current and to eliminate paths other than grid lead insulators 37 and 38 for grid-to-cathode leakage current. For this purpose, two slots or identations 41 and 42 are formed in member 32 through which leads 26 and 21, with insulators 26 and 27 therearound, extend with spaced relation to member 32. The indentations 41 and 42 define the necessary spacing between member 32 and insulators 26 and 27.

Barrier means to prevent loss of grid control through the openings around insulators 26 and 27, which otherwise afford a rather direct path from cathode 8 to anode 22 over which ionization can occur oven when the control grid is negative, including two grid barriers or gussets 43 and 44, are bonded around indentations 41 and 42 respectively, extending toward heat shield 9 as shown. Further, the barrier means include barrier walls and 46 interspaced between gussets 43 and 44 and open ing 15, provided by welding two angle pieces to header 14 as shown. The resultant labyrinthine paths from cathode 8 to anode 22 around walls 45 and 46, around gussets 43 and 4-4, and through the spacing defined by indentations 4i and 42, are so curved and indirect that there is little or no likelihood of discharge initiation occurring over them when the control grid itself is at or below the potential at which ionization occurs through grid wires 30. While the general operation of the thyratron discharge device is conventional and well understood by those skilled in the art, it may be briefly outlined as follows:

A suitable heating current is caused to flow through cathode 8 to heat the emissive substance thereon to a temperature at which it freely emits electrons. This is effected by connecting a source of electric current, preferably a center-tapped transformer secondary winding, across connectors 5 and 7 and heating current then follows a circuit given hcreinbefore. Anode 22 is made positive in potential with respect to cathode 8 by applying a voltage source between connector 5, or preferably the center tap of the above-mentioned secondary winding, and either connector 2 or 6. Either connector 2 or 6 serves as the anode terminal since they are preferably connected together in the bottom of envelope 1 by a jumper 4-7 to divide anode current between leads 20 and 21. The control grid is made negative in potential with respect to cathode 8 by application of a negative voltage between connector 5, or again preferably the above-mentioned center tap, and either connector 3 or 4.. For each value of voltage applied between cathode 8 and anode 22, there is a critical value of control grid voltage at which ionization of the gas and anode current conduction begins; if the voltage is more negative, conduction cannot begin, and if it is more positive, ionization of the enclosed gas takes place and anode current flow continues until the anode voltage is reduced below the plasma voltage, i. e., a very small voltage just necessary to sustain ionization and current flow.

When the control grid is below its critical potential, were it not for the barrier means provided by gussets 43 and 44, and barrier walls 45 and 46, electrons emitted by the cathode might still see the positive anode potential at the anode 22 or anode leads 20 and 21, and be attracted to that potential over the direct path indicated by dashed line 48 with sufficient velocity to cause ionization of the gas and anode current conduction. However, with gussets 41 and 42 and barrier walls 45 and 46 in place as shown, the path from cathode 8 to anode 22 through the spacings defined by indentations 41 and 42 is an indirect curved path as indicated by dashed line 49, the emitted electrons from the cathode do not see anode potential, and hence do not follow the path 49 with enough velocity to cause ionization and loss of grid control. The discharge through the device is thus limited to the direct path through grid wires 30 wherein the negative potential of the control grid is fully effective in con trolling the initiation of conduction.

It has also been found that during operation of the device, barium or other conductive materials are vaporized from the cathode and tend to be condensed on the upper portions of insulators 26 and 27, thereby forming a conductive layer thereon. Anode potential is then transferred down along the surface of the insulators until it is in close proximity to the cathode. In the present device, however, the labyrinth formed by barrier walls 45 and 46 and gussets 43 and 44 is effective in keeping vaporized emissive material away from the surfaces of insulators 26 and 27 in the region between the control grid and the heat shield 9, so that such a conductive layer is not condensed thereon in appreciable quantity. Thus, in the present device, undesirable ionizationand discharge from the cathode to anode lead insulators is eliminated.

During discharge in the device, the gas or vapor therein is in an ionized state, consisting of a plasma of positive gas ions and negative electrons, especially in the direct path between cathode 8 and anode 22 through grid wires 30. Positive ions tend to collect on the surfaces of insulators 26 and 27, if shielding straps 28 and 29 are not present, because they cannot be neutralized by electrons flowing through the insulating material. The charge may; collect and creep down the surface of the insulators until a positive ion charge at anode potential'is present at the bottom of insulators 26 and 27. Highly undesirable ionization and conduction might then be initiated in the bottom portion of envelope 1 between the anode leads or the insulators thereon and the connectors or leads at cathode potential, such a conduction being similar to that in a cold cathode gas tube. It has been found that this breakdown between anode leads and leads at cathode potential when ions collect on the bottom portion of insulators 26 and 27 results from the fact that there are relatively long discharge paths between the two, affording a mean free path long enough to cause ionization of the gas. The same sort of discharge does not occur over the shorter paths from insulators 26 and 27 to the heat shield 9 because these paths are so short that electrons traveling them are not very likely, in the rarified gas, to strike a gas molecule and cause ionization by collision. It is the long paths from anode to cathode potential over which undesirable breakdown may occur since an electron traveling a long path has more chance of striking one of the randomly spaced gas molecules. The problem is to prevent ion charge from collecting along the surface of insulators 26 and 27 to points in the lower region of envelope 1 where such long path discharges may occur.

This'problem has been successfully solved by the use of conducting shielding straps 28 and 29 which permit the neutralization of positive ions as they collect on the top end of insulators 26 and 27 and travel down to the straps. The neutralization of these ions by electrons constitutes anode to cathode current flow, and is therefore not objectionable; at the same time, ionization breakdown of the gas in the envelope is prevented in regions other than the cathode-anode space. The employment of shielding straps 39 and 40 on grid lead insulators 37 and 38 serves the same shielding purpose as that of straps 28 and 29, although it will be understood that the problem is not so critical in the case of the grid lead insulators since the grid to cathode voltage is normally much smaller than the anode to cathode voltage. In addition, the shielding straps 28, 29, 39 and 40 around insulators 26, 27, 37 and 38 respectively contribute to the structural stability of the thyratron electrode construction. The straps, formed around the lead insulators and welded to the heat shield structurally tie all of the electrodes together so that a rigid composite electrode structure effectively supported from all of the connectors is provided.

While the present invention has been described by reference to a particular embodiment thereof, it will be understood that numerous modifications may be made by those skilled in the art without actually departing from the invention. I, therefore, aim in the appended claims to cover all such equivalent variations as come within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An electric discharge device comprising a cathode, a heat shield around said cathode having an opening therein, an anode spaced from said opening in said heat shield, a control grid interspaced between said opening and said anode, at least two anode leads supporting and connecting said anode and extending in proximity to said control grid, said control grid having indentations therein defining spacing between said anode leads and said control grid, and cooperating barrier means supported respectively from said heat shield and said control grid and inwardly of the marginal portions thereof to prevent electric discharge between said cathode and said anode over a path including said spacing between said control grid and said anode leads.

2. An electric discharge device of the thyratron type comprising a cathode, a heat shield surrounding said cathode and having a discharge opening therein, an anode mounted in spaced relation from said opening in said heat shield, a control grid interspaced between said opening and said anode, two anode leads supporting and connecting said anode and extending along the sides of said heat shield, two tubular insulators one around each of said anode leads insulating each lead from said heat shield, and two elongated shielding and supporting straps each positioned around one of said insulators and bonded to said heat shield along substantially its whole length and extending over corresponding lengths of said insulators.

3. An electric discharge device of the thyratron type comprising a cathode, a heat shield around said cathode having an opening therein, an anode spaced from saidopening in said heat shield, a control grid interspaced between said opening and said anode, two anode leads supporting said anode and extending along the sides of said heat shield in proximity to said control grid, two tubular insulators one on each of said anode leads, two shielding straps one around each of said insulators and bonded to the side of said heat shield, said control grid having indentations therein defining spacing between said anode leads and said control grid, and cooperating barrier means supported respectively from said heat shield and said control grid and inwardly of the marginal portions thereof to prevent electric discharge between said cathode and said anode over a path including said spacing between said control grid and said anode leads.

4. An electric discharge device of the thyratron type comprising a cathode, a heat shield around said cathode having an opening therein, an anode mounted in spaced relation to said opening, a control grid mounted in interspaced relation between said opening and said anode, at least two anode lead members extending in insulated relation along the side of said shield to support and connect said anode, at least two control grid lead members extending in insulated relation along the sides of said shield to support and connect said control grid, said control grid having indentations defining spacing therefrom to said anode lead members, gussets around said indentations extending toward said shield, and shielding walls extending from said shield toward said control grid spaced in between said opening and said gussets.

5. An electric discharge device of the thyratron type comprising an hermetically sealed envelope, a cathode supported within said envelope from one end thereof, a heat shielding tank surrounding said cathode and defining a discharge opening in one end of said tank opposite said one end of said envelope, an anode mounted in spaced relation with said one end of said tank, at least two anode lead members supporting said anode and extending from said one end of said envelope in insulated relation to said tank along the sides of said tank, a control grid interspaced between said one end of said tank and said anode, at least two control grid lead members supporting said control grid and extending from said one end of said envelope in insulated relation to said tank along the sides of said tank, said control grid having indentations therein defining spacing between said control grid and said anode lead members, gussets on said control grid around said indentations extending toward said one end of said tank, and barrier walls on said one end of said tank located between said gussets and said opening and extending toward said control grid.

6. An electric discharge device of the thyratron type comprising a generally cylindrical gas-filled hermetically-sealed envelope, :1 heat conserving cylinder mounted within said envelope from one end of said envelope and defining a discharge opening in one end of said cylinder opposite said one end of said envelope, a cathode within said cylinder, four lead members circumferentially spaced around and extending parallel to the surface of said cylinder, four tubular insulators positioned each on one of said lead members, four shielding straps each positioned around one of said insulators and bonded to said cylinder, an anode mounted in axially spaced relation with respect to one end of said cylinder from a first pair of said four lead members, a control grid mounted in axially interspaced relation between said anode and said one end of said cylinder from a second pair of said lead members, said control grid having indentations therein defining spacing between said control grid and said first pair of lead members, gussets around said indentations on said control grid extending axially toward said cylinder, and barrier walls radially interspaced between said opening and said gussets extending axially from said one end of said cylinder toward said control grid.

7. A single ended electric discharge device of the thyratron type comprising a cathode, a generally cylin drical shield surrounding said cathode and having an opening in one end thereof, an anode spaced axially from said one end of said shield and a control member interposed between said anode and said one end of said shield and having a larger diameter than said shield, means for supporting said anode in insulated relation with respect to said shield and said control grid including a pair of conductors extending along the sides of said shield, tivo tubular insulators, one around each of said conductors throughout substantially the entire length thereof, said control grid having passages formed therein of substantially larger dimensions than said insulators to provide a clearance space around said insulators and baffle means supported from said grid and from said shield to provide a path from the opening in said shield to said anode and including the space around said insulators at said control grid.

8. An electric discharge device of the thyratron type comprising a generally cylindrical gas-filled hermeticallysealed envelope, a heat conserving cylinder mounted within said envelope from one end of said envelope and defining a discharge opening in one end of said cylinder cpposite said one end of said envelope, a cathode within said cylinder, a plurality of lead members circumlen entially spaced around the surface of said cylinder, a

plurality of tubular insulators positioned each on one of said lead members, a plurality of shielding straps each positioned around one of said insulators and bonded io said cylinder, an anode mounted in axially spaced relation with respect to one end of said cylinder from said lead members, a control grid mounted in axially interspaced relation between said anode and said one end of said cylinder, said control grid having indentations therein defining spacing between said control grid and said lead members, gussets around said indentations on said control grid extending axially toward said cylinder, and barrier walls radially interspaced between said opening and said gussets and extending axially from said one end of said cylinder toward said control grid.

9. An electric discharge device comprising an envelope and an electrode structure contained in said envelope including a cathode, a heat shield around said cathode having an opening therein, an anode spaced from said opening in said heat shield, a control member interspaced between said opening and said anode, and having the marginal portions disposed in close proximity to the walls of said envelope, at least two anode leads supporting said anode and extending along the sides of said heat shield, an elongated tubular insulator on each of said anode leads, a shielding strap around each of said insulators and bonded to the side of said heat shield, said straps extending over substantially the Whole lengths of said insulators, said control member having marginal indentations receiving said anode leads and providing required spacing therebetween, and cooperating barrier means supported respectively from said heat shield and said control member and extending oppositely intermediate said indentations and said opening in said heat shield to prevent electric discharge between said cathode and said anode over a path including said spacing be tween said control member and said anode leads.

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